I'm intalling a plastic 36" x 36" shower in a tight space. Plumber says that backer-board isn't necessary to install behind shower walls onto studs. He says that he can simply to install shower onto 2x4 studs of the walls and it will be ok. Is that ok to install a plastic (quite hard plastic probably 1/32 to 1/8 of an inch thickness) shower directly onto studs or should a backer-board be installed in between shower walls and wall studs?? How about vapor barrier? all the walls are inside walls. Thanks for your help.